1. Field of the Invention
This invention relates to hydrogenation catalysts and to a hydrotreating process for removing sulfur contaminants from hydrocarbon feedstocks. More particularly, this invention relates to a novel catalytic material useful for the hydrodesulfurization of hydrocarbons, preferably petroleum oils containing residual hydrocarbon components and having a significant metals content, and to hydrotreating processes employing such catalysts.
2. Description of the Prior Art
The current trend in refinery crude slates is for the utilization of increasingly heavy and "dirty" feedstocks, which contain large amounts of sulfur, nitrogen, metals, etc. In addition, an increasing proportion of these crude oils is present as residual fuels, and the principal outlet for these fuels is as low sulfur fuel oils subsequent to catalytic desulfurization. Because of the increasing problems of air pollution, particularly with regard to sulfur oxide emissions, increasing concern among refiners has arisen with respect to the utilization of these feedstocks. Consequently, the development of an efficient and economic means for sulfur removal from these heavy, sulfur-bearing oils has become a primary research goal in this industry.
Various methods for removal of sulfur from these feedstocks have been the subject of intensive research efforts by this industry. At present, the most practical means of desulfurizing such heavy oils is catalytic hydrogenation at elevated pressures and temperatures in the presence of an appropriate catalyst. While these methods are relatively efficient in the case of certain distillate oils, they become less efficient as increasingly heavy feedstocks, such as whole or topped crudes or residua, are processed.
Difficulty has been experienced in achieving an economically feasible catalytic hydrodesulfurization process because, notwithstanding the fact that the desulfurized products may have a wider marketability, the manufacturer may be able to charge little or no additional premium for the low sulfur products and hydrodesulfurization operating costs have tended to be relatively high in view of the previously experienced, relatively short life for catalysts used in hydrodesulfurization or residua-containing stocks. Short catalyst life is manifested by inability of a catalyst to maintain a relatively high capability for desulfurizing charge stocks with increasing quantities of coke and/or metallic contaminants which act as catalysts poisons. Satisfactory catalyst life can be obtained relatively easily with distillate oils but is especially difficult to obtain in desulfurizing petroleum oils containing residual components since the asphaltene or asphaltic components of an oil, which tend to form disproportionate amounts of coke, are concentrated in the residual fractions of a petroleum oil, and since a relatively high proportion of the metallic contaminants that normally tend to poison catalysts are commonly found in the asphaltene components of the oil.
The most common desulfurization catalyst is cobalt molybdate on a alumina base. However, any of the group VI and group VIII metals may be employed as a hydrogenation component on a suitable refractory base material. Typical operating condition ranges for resid and/or crude desulfurization are a temperature of about 650.degree. to 850.degree. F., a space velocity of about 0.1 to 5.0 LHSV, a pressure of about 500 to 3000 psig and a hydrogen circulation rate of about 1000 to about 15,000 scf/barrel of feed. This type of process has been operated in such a manner as to maintain a substantially constant conversion for level of sulfur removal. In order to achieve this desired level of sulfur removal, the operating conditions are steadily increased in severity to compensate for the gradual catalyst deactivation primarily due to metals poisoning and coking.
Process severity may be described as being directly related to temperature and pressure and inversely related to the space velocity of the process. Thus, in order to increase severity, one might increase pressure and/or temperature or decrease the space velocity. As most process units are sized based on throughput and pressure, neither the contact time nor the pressure can be significantly increased. Therefore, severity is typically increased through a temperature increase. Thus most residua desulfurization reactors are initially operated at a "start of run" temperature of about 650.degree. F. to 750.degree. F. As the desulfurization catalyst activity decreases due to metals deposition and coke formation, the reaction severity is increased by increasing the temperature so as to maintain a desired, substantially constant sulfur removal level. "End of run temperature" is typically about 800.degree. F. and is reached when the catalyst activity has been significantly decreased, e.g., due to metals poisoning and coking. Were it not for such metals poisoning of the desulfurization catalyst, the operating cycles could be lengthened, or the severity could be reduced (lower temperatures and/or lower pressures and/or higher space velocities).
One of the great difficulties in the desulfurization of heavy oils such as residua is that the asphaltenic components contained in the resid are of a type that are difficult to desulfurize. In addition, the high metals content present in those alphaltenic structures acts as a contact solids poison which acts primarily by blocking up the pores near the external surface of the contact solids so that the internal surface becomes unavailable to carry out the desulfurization reaction; the life of the desulfurization contact solids is limited by metals deposition in the pore structure of the solids.
Although metallic contaminants, existing as oxide or sulfide scale may be removed, at least in part, by a relatively simple filtering technique, and the water soluble salts are at least part removable by washing in a subsequent dehydration procedure, a much more severe treatment is required to effect the destructive removal of the organo-metallic compounds. However, the higher molecular weight organo-metallic molecules in these feedstocks can only be broken down when operating under operating conditions more severe than needed for desulfurization, which also tend to accelerate catalyst dactivation due to accelerated coke and metal deposition on the catalyst surfaces.
Therefore, it has been suggested that metals removal prior to treatment of non-metallic impurities such as sulfur is indispensable. Simultaneous treatment of the hydrocarbon for removal of all these impurities withouth pretreatment for metals removal requires an amount of catalyst in large excess to the theoretical amount required for desulfurization. Because catalysts for these desulfurization operations are very expensive, inexpensive demetallation catalysts having excellent demetallation characteristics have been sought.
When demetallation treatment is carried out beforehand, hydrocarbons are treated by using either an ordinary or high porosity desulfurization catalyst or a waste catalyst having almost no desulfurization activity or by using bauxite, red mud and the like as the catalyst in a so-called guard reactor. All these catalysts, however, have defects in that either the activity of demetallation is low or the life of the catalyst is too short and, moreover, they are very unsatisfactory for the purpose of carrying out a selective and effective demetallation reaction.
In the case of a catalyst having a relatively high demetallation activity, the desulfurization reaction also proceeds simultaneously. The demetallation reaction, like the desulfurization reaction, is a hydrogenation reaction which is carried out in the presence of a catalyst under hydrogen pressure and at a high temperature. The demetallation reaction commonly takes place together with the desulfurization reaction since metals are deposited on the active catalyst sites during the desulfurization reaction. In desulfurization treaments using conventional desulfurization catalysts, the higher the desulfurization is raised, the higher the demetallation becomes. The desulfurization and demetallation reactions take place in an almost definite proportion under the same conditions. Even when demetallation is carried using the coventional desulfurization catalyst, it is totally impossible to avoid the desulfurization reaction which takes place in the definite proportion.
At the present time and certainly for several years into the foreseeable future, low sulfur fuel oils are and will be in critical demand. At the same time that recent legislation has reduced the allowable sulfur levels in fuel oils, the overall demand for fuel oils has increased markedly. As a consequence, the need for desulfurized petroleum products such as fuel oils has been doubly increased.
An object of this invention is to provide a method for the hydrodesulfurization of metals and sulfur containing petroleum oils, preferably those containing residua hydrocarbon fractions, whereby the operating cycle, that is, the number of days on stream, for such a process may be significantly increased without any significant decrease in sulfur removal. An additional object of this invention is to provide a method for hydrodesulfurizing petroleum oils, preferably those containing residua hydrocarbon fractions, whereby the severity of the operation and the attendant investment in operating costs are decreased. Another object of this invention is to provide a hydrodesulfurization method and catalyst whereby the metals poisoning of the desulfurization catalyst is significantly reduced. Other and additional objectives of this invention will become obvious to those skilled in the art following a consideration of the entire specification including the claims.